This application relates generally to fiber-optic communications and more specifically to a method and apparatus for retroreflecting optical signals.
Fiber-optics-based telecommunication systems are increasingly viewed as a means for addressing the recent explosion in global demand for bandwidth. Such systems are beginning to deploy dense wavelength division multiplexing (xe2x80x9cDWDMxe2x80x9d) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is thus multiplied by the number of DWDM channels used.
In order to perform wavelength routing functions, a variety of optical elements, including focusing elements such as lenses or mirrors, dispersion elements such as diffraction gratings or prisms, and direction elements such as retroreflectors, may be used. For certain types of routing, an array of retroreflectors may be used, the performance of which is affected by a number of factors, including sensitivity to temperature. Because of the environments in which wavelength routers are deployed, they may experience temperature changes of up to about 55xc2x0 C. As a result, components used in the retroreflectors may change in size and affect the operation and overall efficiency of the router. For example, such temperature changes may affect the focal length of the retroreflectors and/or cause signals to drift off targets. In addition, some designs for wavelength routers may include a relatively large number of retroreflectors so that designs that limit the amount of material used without adversely affecting performance are generally preferable.
Hence, in different aspects, this invention is related to a method and apparatus that provides an athermalized retroreflector and/or limits the amount of material needed to fabricate the retroreflector. In this way, an improved wavelength router is provided that may successfully be deployed in a wide variety of environments.
In one embodiment, a substrate is provided over which a moveable micromirror is formed. The moveable micromirror is adapted to be tilted into at least two distinct positions. A multimirror stack is also positioned on the substrate. The multimirror stack includes a plurality of fixed structures bonded together. Each of the fixed structures has a reflective surface, and the multimirror stack is configured with respect to the moveable micromirror to provide retroreflection paths. An optical ray incident at a predetermined angle with respect to the substrate is subjected to retroreflection for at least two distinct positions of the moveable micromirror. Each retroreflection includes a reflection off the moveable micromirror and a reflection off one of the fixed structures. In certain embodiments, the fixed structures are substantially isomorphic in shape. They may also comprise the same material, which in one embodiment is PYREX(copyright). The fixed structures may be bonded along surfaces that are substantially parallel to the substrate.
In another embodiment, an additional fixed structure having a reflective surface is positioned on the substrate and each retroreflection also includes a reflection off this additional fixed structure. Such an embodiment permits the optical ray to be incident substantially orthogonal to the substrate. Accordingly, in one embodiment, a window is provided for hermetically sealing the moveable micromirror, the multimirror stack, and the additional fixed structure.
Embodiments of the invention may include a plurality of such retroreflectors as part of a wavelength router. Such a wavelength router is configured to receive, at an input port, light having a plurality of spectral bands, subsets of which are directed to respective output ports. An optical train is disposed between the input port and the output ports providing optical paths for routing the spectral bands. The optical train includes an optical element disposed to collimate light emanating from the input port and a dispersive element disposed to intercept light traveling from the input port. The retroreflectors lie in a focal plane of the optical element and each is adapted to intercept a spectral band and direct the spectral band back towards the optical element.